Many photoconductive detectors now use the material Cadmium Mercury Telluride (CMT) as the photosensitive semiconductor material and have contacts of gold, indium, or aluminium metal. See for example UK Patent No. 1,488,258 (U.S. Pat. No. 3,995,159). Conventionally, photoconductive detectors have a rectilinear geometry--ie the semiconductor material is provided in square or rectangular shape and the contacts are provided at the ends of the material and are patterned each with an orthogonal straight edge boundary between the metal and the semiconductor material. Some detectors--particularly those incorporated in integrated arrays, may be provided with contacts not of metal but of heavy doped semiconductor material--i.e., contact, may be formed at the interface between light doped or intrinsic photosensitive semiconductor material and regions of heavy doped conductive material--so-called "light-heavy" (l-h) contacts.
Photoconductive infra-red detectors of CMT material, particularly of the high purity CMT material now available, have the property of long excess carrier (ie photocarrier) lifetime. Typical bulk lifetimes are between 1 to 4 .mu.s (8 to 14 .mu.m band sensitive CMT material and between 10 and 20 .mu.s (3 to 5 .mu.m sensitive CMT material). This makes detectors particular susceptible to accumulation effects. The devices, in the absence of accumulation effects would normally be operated in a sweep-out condition--where the effective excess minority carrier lifetime is determined by the transit time of minority carriers through the device, which is very much less than the bulk lifetime--thus the delayed recombination of excess carriers at the contact, ie accumulation, results in an increased effective lifetime. [The phenomenon of carrier accumulation in semiconductors was first suggested by Low (Proc Phys Soc Lond B68, 310 (1955)) and the theory has been developed by Gunn (Jnl Electronics & Control 4, 17 (1958)].
Carrier accumulation has two consequences for detector performance. Firstly, the detector responsivity (this is defined as the voltage (or equivalent voltage) output corresponding to a radiation flux of 1 watt on the detector, or in the case of three lead structures eg Patent No. 1,488,258, a radiation flux of 1 watt per detector width squared) is increased. This follows because the time spent by excess carriers in the detector is increased, i.e., the sweep-out time is lengthened. Secondly, however, the frequency response of the detector is degraded. For the detector described in UK Patent No. 1,488,258, this latter consequence is manifest as a degradation of the spatial resolution afforded by the detector.